A technique has been known for installing a wavelength selector device in an optical resonator and coating one end of a YAG crystal with a tri-chromacy coating in order to produce oscillation at a wavelength of 1.06 μm and oscillation at a wavelength of 1.32 μm simultaneously in the YAG crystal through excitation with a semiconductor laser and selectively promote the latter oscillation while suppressing the former oscillation (for example, as shown in Patent Citation 1).
Also, another technique has been known for installing an etalon in an optical resonator and modifying the temperature of the etalon so as to match the transmission peak of the etalon with the peak wavelength of the laser output (for example, as shown in Patent Citation 2).
A further technique has been known for, when two or more longitudinal modes are provided for wavelength conversion with a nonlinear optical crystal installed in an optical resonator, minimizing the generation of mode competition noise caused by from coupling of the modes through sum frequency generation can be done by making the longitudinal mode into a single mode (for example, as shown in Non-patent Citation 1).
Also, a still further technique has been known for controlling the oscillation mode through creating a second resonator in the optical resonator (for example as shown in Non-patent Citation 2).    Patent Citation 1: Japanese Patent Laid-open Publication No. (Showa) 64-31485.    Patent Citation 2: Japanese Patent No. 3509598.    Non-patent Citation 1: J. Opt. Soc. Am. B. Vol. 3, No. 9, P 1175 (1986).    Non-patent Citation 2: Lasers at p 534 (University Science Books, Mill Valley, Ca., 1986).